With the advent of the Internet it has become increasingly popular to transmit or broadcast digital data over wireless links. The use of any communication medium is constrained by Shannon's law, which states that the amount of information that can be transmitted is proportional to the bandwidth available. There is a limited amount of spectrum available that does not have undesirable characteristics—such as line of sight restrictions, or excessive attenuation in rain or fog. Typical approaches to reusing spectrum involve directional transmission/reception, use of low power cells, frequency hopping, or coding. While it is theoretically possible to transmit more than 1 bit/sec per Hertz of bandwidth (the unit is often abbreviated to “bits per Hz”), in practice this is often close to the designed-in ratio for many wireless data systems. However, the desire for broadband (high bit rate) connections is pushing the industry into exploring ways to increase this ratio and conserve on spectrum. For example, the IEEE 802.11a standard prescribes 64 QAM as the modulation technique in high bandwidth mode, which is equivalent to 6 bits/sec/Hz. The ability to increase this number is limited by the signal to noise (S/N) ratio at the receiver, as this affects the receivers ability to distinguish between the different symbols in the symbol constellation. The relationship is typically logarithmic, meaning that for every additional bit/sec/Hz we have to double the S/N ratio. The effect on a wireless system is to reduce coverage area, or increase the maximum required signal strength.
Another effect of the use of digital transmissions is that there is a very sharp cutoff—upto a certain point noise has no effect on the signal, but beyond a certain range the signal quickly becomes so garbled as to be completely useless. The sharpness of this cutoff is enhanced by the fact that many digital transmissions are organized into message blocks, and the whole block is discarded if any uncorrectable error is found. For digital TV broadcasts this is considered desirable, you either have a good picture or nothing. But for other types of uses this is undesirable; people browsing the Internet, emergency personnel or police might be willing to live with a slower or lower quality connection, but being cutoff completely is disastrous.
There is also another effect with high bits/sec/Hz digital systems—because the signal is pretty high above the noise floor at the cutoff range, there is a large zone beyond the cutoff range where reception at the same frequency of the transmitter is not possible. At 20 bits/sec/Hz this zone can be 400 times the area of the zone where reception is possible.
Compare this with an analog broadcast. As you get further and further from the transmitter, you lose clarity, but you can still recover significant amounts of information from the transmission. This does not require any coordination with the transmitter—it happens automatically. Many techniques have been implemented to adapt the rate of digital transmission to conform to a value suitable for adequate reception at the receiver, but all rely on negotiation between the transmitter and receiver. Moreover, while the low bitrate transmission is in progress, no other transmission can take place. For example, in 802.11a, one changes the modulation technique from 64 QAM all the way down to BPSK as the received SN ratio decreases.